One of the traditional problems facing gravure ink formulators has been low solution viscosities. These low viscosities make it difficult to formulate inks having the desired pigment-to-binder ratios at press viscosity that are necessary for this type of printing.
The high acid values common in rosins may be lowered by reacting them with zinc oxide, calcium acetate, or similar compounds to produce metallic resinates. Metal resinates are widely used as binders in gravure printing due to their rapid solvent release and their ability to prevent some basic pigments from livering. These resinates are soluble in hydrocarbons, but not in alcohol.
However, one of the shortcomings of using metal resinates in gravure inks has been their low solution viscosities (a byproduct of their low molecular weights). These low viscosities lead to the same printing ink formulation problems referenced above.
A critical property of ink formulations that is directly linked to viscosity is dilutability. Viscosity is measured by the time required for an exact quantity of solution to flow by gravity through a specially sized apparatus. Dilutability is measured by the amount of solvent needed to reduce the viscosity of a given weight of resinate solution to a certain level. The typical specification calls for the volume of toluene needed to reduce 100 grams of resinate to 18 seconds as measured with a #2 Shell cup. While common dilutability values of commercial resinates are between 70 to 120 ml., ink makers would like dilutabilities higher than this in order to be able to achieve a desirable balance of solids, color strength, and viscosity in finished inks.
To solve these problems of viscosity and dilutability, ink makers traditionally have added small amounts of ethylcellulose (E/C) or ethylhydroxyethylcellulose (EHEC) to the ink as a dilution enhancer (the properties of which are discussed in U.S. Pat. No. 2,610,180). EHEC owes its thickening ability to three factors: (a) a very high molecular weight, (b) a rigid molecular structure, and (c) intermolecular association via hydrogen bonding of unetherified hydroxyl groups on the cellulose backbone. In this way E/C or EHEC can be used to produce a drastic reduction in resin solids at press viscosity (expressed in the industry as a "high dilution").
However, the use of E/C or EHEC may give rise to other problems, as described in Leach, R. H., The Printing Ink Manual, Van Nostrand Reinhold Co. Ltd., London, 1988. Very small amounts of these compounds can cause substantial reductions in the gloss of an ink. Moreover, these compounds are relatively expensive. Finally, E/C and EHEC have demonstrated resinate incompatibilities which seem to vary from batch to batch, and may manifest itself in phase separation of the ink upon standing.
Attempts have been made to avoid those problems by producing other binders which can be used as dilution enhancers. For example, U.S. Pat. No. 3,468,829 teaches the preparation of printing ink binders based on reaction products of: 1) natural resins, 2) other synthetic resins (for example, hydrocarbon resins having a bromine number of 5 to 80), and 3) calcium compounds (and, if appropriate, other compounds of group II of the periodic system). The patent also teaches reacting an unsaturated dicarboxylic acid (i.e., maleic anhydride) at the same time. However, this process has a disadvantage in that the resulting viscous products are virtually not utilizable as printing ink binders.
Other known binders are taught in U.S. Pat. Nos. 4,528,036 and 4,552,592. Here, the binders are based on the reaction products of: 1) natural resins, 2) a copolymer or a synthetic resin, and 3) calcium compounds. The resulting reaction products are reacted subsequently with acetic acid to form a salt of the corresponding resin. However, these solutions are of relatively low molecular weights and viscosities.
Yet another known binder is the product complex formed by reaction of a metal resinate and an amine-reactive polyamide as taught in U.S. Pat. No. 4,767,835. The amine-reactive polyamide, being a condensation polymer, is of comparatively low molecular weight.
U.S. Pat. No. 4,289,678 teaches an acrylic thickener for use in formulating gravure inks. This thickener, the reaction product of an addition polymerization reaction, has a molecular weight in the range of 100,000 to 2,000,000. This large molecular weight may cause incompatibility problems for ink formulators similar to those exhibited with E/C and EHEC.
Commonly assigned U.S. Pat. No. 5,098,479 teaches dilution enhancing aminoacrylate resin binders. However, as amine-containing monomers are employed in the production of these binders, they are primarily suitable for use in formulating zinc-containing metal resinate inks (i.e., they give little or no dilution enhancement in zinc-free resinates).
Despite the development of these, and other, thickeners E/C and EHEC remain the preferred viscosity extenders in the field. Therefore, it is an object of this invention to produce economical, dilution enhancing binders for use in formulating gravure printing inks.
Another object of this invention is to produce hydrocarbon-soluble binders which, when used to formulate gravure printing inks, enhance both the gloss and the color properties of the inks.
A further object of this invention is to produce gravure printing ink formulations with enhanced gloss and color characteristics.